1. Field of the Invention
This invention relates generally to a television receiver that can be used in connection with a so-called high definition television (HDTV) system, and more particularly is directed to an apparatus for converting video signals of a first format, for example, the HDTV format, to video signals of a second format, for example, that of the standard NTSC video signals.
2. Description of the Prior Art
In order to provide a higher resolution than that achieved with the existing standard television systems, a high definition television (HDTV) system has been developed in which the number of horizontal scanning lines in each field or frame is increased and the frequency band of the video signal is widened, and such high definition television system is now being perfected and made practical. Thus, for example, a standard NTSC television receiver employs 525 horizontal scanning lines and the frequency band of the NTSC luminance signal is 4.2 MHz, while the HDTV television receiver employs 1125 horizontal scanning lines and the HDTV luminance signal has a frequency band of 20 MHz.
Furthermore, although the aspect ratio of the picture screen of the existing standard NTSC television receiver, that is, the ratio of its horizontal dimension to its vertical dimension, is 4:3, the corresponding aspect ratio of the picture screen of the HDTV television receiver has been selected to be 16:9, or approximately 5:3. Therefore, even if a so-called scan-converter is employed for changing the number of the horizontal scanning lines in a received HDTV television signal to correspond with the 525 horizontal scanning lines used in the standard NTSC television receiver, the different aspect ratios of the picture displays according to the HDTV and NTSC formats give rise to problems in displaying the scan-converted HDTV video signal on the screen of a standard NTSC television monitor receiver.
As shown for example on FIG. 1A, in order to display, on the picture screen of an NTSC television monitor receiver represented in full lines and having the aspect ratio of 4:3, the full picture corresponding to an HDTV video signal intended to be displayed on the picture screen of an HDTV television receiver having an aspect ratio which, for the sake of simplicity, is assumed to be 5:3, as represented by the dotted lines on FIG. 1A, it has been proposed to subject the HDTV video signal to a picture conversion in which the time base of the video signal is compressed to 4/5th of its original value and the number of horizontal scanning lines is converted from 1125 to 525. However, when such previously proposed picture conversion is effected, the resulting displayed picture is vertically elongated as shown in FIG. 1A so that the displayed picture is unnatural or distorted.
On the other hand, if the HDTV video signal is subjected to a picture conversion in which, while the HDTV aspect ratio of 5:3 is maintained, the picture size is reduced so as to have a horizontal size no greater than that of the picture screen of an NTSC television monitor receiver on which the HDTV video signal is to be displayed, as shown on FIG. 1B, distortion of the picture is avoided, but upper and lower portions of the picture screen are left blank or black, as indicated by the shaded areas. Such blank or black upper and lower portions of the picture screen each correspond to approximately 1/10th the vertical dimension of the picture screen. Thus, of the 525 horizontal scanning lines of the standard NTSC television monitor or receiver, only about 450 horizontal scanning lines are used to form the effective picture area in FIG. 1B. Consequently, although the previously proposed arrangement illustrated in FIG. 1B avoids picture distortion, it does so at the expense of substantially degrading the enhanced vertical resolution which is one of the desirable features of the HDTV system.
In order to avoid both the picture distortion of FIG. 1A and the degraded vertical resolution of FIG. 1B, it has been proposed to display a HDTV video signal as a natural looking image, for example, as shown on FIG. 1C. In accordance with the proposal illustrated by FIG. 1C, left- and right-hand side portions of the picture that would be displayed on the picture screen of an HDTV television receiver, that is, a picture screen having an aspect ratio of 5:3 as shown in dotted lines on FIG. 1C, in response to an HDTV video signal, are omitted when such video signal is displayed on the picture screen of an NTSC television monitor receiver having an aspect ratio of 4:3, and which is represented in full lines on FIG. 1C. In this case, however, if the right- and left-hand side portions of the HDTV television picture contain important video information, such video information will be omitted from the picture displayed on the picture screen of the standard NTSC television monitor receiver and cannot be observed or displayed.